PARTICLE-SCALE INVESTIGATION OF SEEPAGE INDUCED GEOTECHNICAL INSTABILITY

土木工程经常会遇到水从地面流过。例子包括堤坝、防洪墙和堤防、地下挖掘、隧道和深地下室。在考虑设计时，工程师力求避免渗水所施加的浮力足以使土壤的有效应力降至零，从而导致隆起破坏或“流沙”。考虑到土壤是一种连续但多孔的材料，可以确定这种临界情况。然而，土壤是由不同大小和形状的单个颗粒组成的。人们越来越认识到，施加在颗粒上的渗流力可以优先侵蚀沙质土壤中较小的颗粒。根据工程实践中使用的经典连续介质计算，在被认为是安全的情况下，可能存在显著的土壤内侵蚀；这种现象被称为内部不稳定。我们将通过研究所涉及的基本颗粒尺度机制，提高对内部不稳定性的理解，从而提高我们对如何安全设计和评估基础设施的知识。在国际上，几个研究小组正在对这个问题进行相对较大的实验。拟议研究中的粒子尺度重点将是对这些研究的补充，而不是补充。具体的研究方向来源于我们之前的研究，最近发表的其他小组的实验数据，并考虑到最近发布的设计指南（如国际堤防手册）和对岩土工程设计欧洲规范EC7中水力破坏指南的修改建议。这项跨机构提案将结合谢菲尔德大学（UoS）测试透明土壤的实验专业知识和伦敦帝国理工学院（IC）离散元素建模（DEM）的技能。我们将以最先进的形式使用这些技术。在UoS，将开发配备激光光源的测试设施，以实现土壤样品内部颗粒运动的可视化，同时还使用示踪颗粒观察流体流动。在进行这些观察时，可以控制围应力和偏应力的三轴应力路径装置的发展将是本研究的一个特别新颖的方面。IC将继续支持使用高性能计算来实现地质力学DEM模拟，该项目将利用机械工程系最近开展的工作，使DEM模拟与流体流动的计算流体动力学（CFD）建模相结合。UoS和IC一直在独立研究内部不稳定问题，因此该提案标志着及时的合作，以统一他们的互补技能。例如，DEM模型可以提供在实验室中无法测量的颗粒应力信息，而在物理测试中可以直接观察到真实材料的不稳定性，而无需任何数值模型固有的任何理想化和假设。该研究将澄清：(i)哪些材料最初易受体积变化的内部不稳定性影响，以及材料最初以恒定体积侵蚀（即颗粒结构的沉降或崩溃）过渡到具有体积变化的条件。（ii）渗流速度或水力梯度与侵蚀起始的相关性较好。（三）应力水平如何影响敏感性；特别考虑了应力各向异性以及主应力方向与渗流方向的关系。

Civil engineering works often encounter water flowing through the ground. Examples include embankment dams, flood walls and embankments, excavations beneath the water table, tunnels and deep basements. When considering their design, engineers seek to avoid cases where the buoyancy forces exerted by the seeping water are sufficient to reduce the effective stress in the soil to zero, resulting in heave failure or "quicksand". This critical scenario is identified by considering the soil to be a continuous, but porous material. However soil is made up of individual particles of varying size and shape. Awareness is growing that seepage forces imparted on the particles can preferentially erode the smaller particles in sandy soils. There can be significant internal erosion of the soil under scenarios that are considered safe according to the classical continuum calculations used in engineering practice; this phenomenon is called internal instability. We will improve understanding of internal instability and thereby our knowledge of how to design and assess infrastructure safely, by studying the fundamental, particle scale mechanisms involved. Internationally, several research groups are undertaking relatively large experiments of this problem. The particle-scale emphasis in the proposed research will complement, rather than supplement, these studies. The specific research direction originates from our prior research, recently published experimental data from other groups, and consideration of recently published design guidelines (e.g. the International Levee Handbook) and the proposed modifications to the hydraulic failure guidelines in the Eurocode EC7 for geotechnical design.This cross-institutional proposal will combine experimental expertise in testing transparent soil at the University of Sheffield (UoS) with skills in discrete element modelling (DEM) at Imperial College London (IC). We will use these techniques in their most advanced form. At UoS testing facilities equipped with a laser light source will be developed to enable visualization of particle movement inside soil samples while also using tracer particles to observe fluid flow. The development of a triaxial stress path apparatus where the confining and deviatoric stress can be controlled while making these observations will be a particularly novel aspect of this research. IC will continue to champion the use of high performance computing to enable geomechanics DEM simulations and the project will exploit recent work that was carried out in the Department of Mechanical Engineering to enable DEM simulations to be coupled with computational fluid dynamics (CFD) modelling of the fluid flow. Both UoS and IC have been working independently to examine the problem of internal instability and so this proposal marks a timely collaboration to unify their complementary skill sets. For example the DEM model can provide information about particle stresses that cannot be measured in the laboratory, while instability can be directly observed for real materials in the physical tests without any of the idealizations and assumptions which are inherent in any numerical model. The research will clarify:(i) Which materials are initially susceptible to internal instability with volume change and the conditions whereby a material that initially erodes at a constant volume (i.e. settlement or collapse of the particle structure), transitions to having volume change. (ii) Whether seepage velocity or hydraulic pressure gradient correlates better with the initiation of erosion.(iii) How the stress level influences susceptibility; particularly considering stress anisotropy and the relation between principal stress orientation and seepage direction.